The present invention relates to a plain bearing, especially the plain bearing having a back metal on whose inner surface a bearing alloy layer and an overlay layer are provided in this order.
There has been known a plain bearing in which a bearing alloy layer is provided on a back metal and an overlay layer is provided on the bearing alloy layer thereby providing the bearing with improved properties of conformability and embeddability for foreign substances, the overlay layer consisting of a soft material such as a Pbxe2x80x94Sn alloy. There has been also known a plain bearing in which an intermediate layer is provided between a bearing alloy layer and an overlay layer in order to prevent a component of the overlay layer from diffusing into the bearing alloy layer and improve adhesiveness of the overlay layer to the bearing alloy layer.
It is important to internal-combustion engines, for example, how to fully reduce an energy loss when transforming kinetic energy from combustion cylinders into rotary motion through a crank shaft. To this end, it is necessary for the crank shaft and bearings to be machined with a high accuracy with regard to the geometry such as straightness and circularity of the crank shaft and the measurements of the bearings. But, since the crank shaft is not a real rigid body, it bends under dynamic rotational motion to cause disorder of measurements. A solution therefor has been to provide a soft material layer on the outermost surface of the bearing.
But, in recent high performance engines, since bearings receive a high surface load, it is impossible to avoid a direct contact between the mating shaft and the bearings due to a change of oil film after wear loss of the soft material layer as a running-in layer which absorbs the disorder of measurements at an initial running stage. Thus, an overlay layer per se suffers progressive wear to soon expose a hard bearing alloy layer. In such a state, if there are contained machining chips, a substance of deteriorated oil and so on in lubricant oil, a seizure will occur at the sliding-contact interface between the shaft and the bearings.
Accordingly, there has been known a plain bearing as shown in FIG. 7, which comprises a bearing alloy layer 61 on whose inner surface grooves (or recesses) 64 are formed and an intermediate layer 62 is provided so as to follow the grooves 64. According to such a layered structure, even if an overlay layer 63 wears to expose the intermediate layer 62, there will appear a state that a soft material of the overlay layer 63 in the grooves co-exists with the exposed intermediate layer thereby ensuring anti-seizure property of the plain bearing.
Since such grooves are, typically, formed spirally by fine boring machining, they extend parallel with one another in a circumferential direction of the bearing so as to have a constant pitch. In operation of the plain bearing, when the overlay layer wears progressively, there will appear a stripe pattern extending circumferentially and consisting of the bearing alloy layer and the overlay layer. In this state, when a foreign substance enters between the plain bearing and the mating shaft, it will be embedded in the overlay layer remaining in the grooves or recesses to exhibit an improvement effect of the anti-seizure property.
With regard to the above plain bearing, in the case where an area fraction rate of the grooves or recesses is relatively small, namely an exposure area of the bearing alloy layer is large, although excellent fatigue resistance and wear resistance can be obtained, the embeddability for a foreign substance is deteriorated resulting in inferior anti-seizure property. On the other hand, if an area fraction rate of the grooves or recesses is made relatively large, the embeddability for a foreign substance will become excellent.
However, because the conventional plain bearing has been provided with uniform grooves or recesses on the inner surface of the bearing alloy layer, there is a limitation in making the plain bearing excellent in fatigue resistance, wear resistance and anti-seizure property.
Thus, the present invention is directed to a plain bearing which is provided with a plural types of circumferential recessed zone, which comprise a zone type of broad width recesses and another zone type of narrow width recesses, on the inner surface of the bearing alloy layer to have enhanced embeddability for a foreign substance while ensuring good fatigue and wear resistance properties.
According to the invention, there is provided the plain bearing comprising a back metal, a bearing alloy layer and an overlay layer, in which the bearing alloy layer and the overlay layer are formed on the inner surface of the back metal in this order, wherein the inner surface of the bearing alloy is provided with a plural types of circumferential recessed zone which comprise a zone type of broad width recesses and another zone type of narrow width recesses, and the recesses are filled with the overlay material.
According to the above structure, fatigue and wear resistance properties can be ensured by the presence of the zone type of narrow width recesses. On the other hand, the plain bearing exhibits enhanced embeddability for a foreign substance by the zone type of broad width recesses.
Typically, the bearing alloy layer is made from a Cu alloy or an Al alloy. The overlay layer may be optionally made from Pb, a Pb alloy, Sn, an Sn alloy, or a resin suitable for a sliding-contact application, e.g. PTFE or PFA.
In order to form the plural types of circumferential recessed zone, which have different area fraction rates with one another, on the inner surface of the bearing alloy layer, the masking-etching method is desirable because of a highest flexibility in designing the recess pattern. The forming method of the plural types of circumferential recessed zone is optional and may be the conventional boring machining by which grooves can be formed spirally on the inner surface of the bearing alloy layer. In the case of the boring machining, a feeding rate of the cutting tool is changed, as a first option, during machining in order to form the plural types of circumferential recessed zone, and a plurality grooves are simultaneously formed with utilization of a plurality of cutting tools, as a second option.
With regard to the overlay layer, it may be formed directly on the bearing alloy layer, or an intermediate layer made of Ni or Ag, for example, may be provided between the bearing alloy layer and the overlay layer to enhance adhesiveness and prevent a component element of the overlay layer from diffusing into the bearing alloy layer. The intermediate layer may be made of an Ni alloy, Cu, a Cu alloy, Co, a Co alloy, Sn, an Sn alloy, an Ag alloy, Zn or a Zn alloy other than Ni and Ag, as occasion demands.
Even if the intermediate layer is made of a hard material and the overlay layer wears to expose the intermediate layer, because the bearing alloy layer is provided with the recesses on the inner surface, the hard intermediate layer is never exposed through the whole exposure surface, whereby anti-seizure property can be ensured.
With regard to arrangement of the plural zone types of circumferential recesses, a narrow width recess and a broad width recess may be arranged alternately or randomly in the axial direction, or two groups of the plural zone types of circumferential recesses, which consist of a plural rows of the narrow recesses and another plural rows of the broad recesses, may be arrange alternately in the axial direction.
According to the invention plain bearing which comprises the back metal, the bearing alloy layer and the overlay layer being formed in this order, and in which the inner surface of the bearing alloy is provided with the recesses of the plural types of circumferential recessed zone in a dispersed manner and the recesses are filled with the overlay material, it is possible to attain excellent anti-seizure property, fatigue resistance and wear resistance.